Froth flotation is a known process for separating valuable minerals from waste material, or for the recovery of finely-dispersed particles from suspensions in water. Typically, an ore as mined consists of a relatively small proportion of valuable mineral disseminated throughout a host rock of low commercial value (gangue). The rock is crushed or finely ground so as to liberate the valuable particles (values). The finely-ground particles are suspended in water, and reagents may be added to make the surfaces of the values non-wetting or hydrophobic, leaving the unwanted gangue particles in a wettable state. Air bubbles are then introduced into the suspension, which is also referred to as pulp or slurry. A frother may be added to assist in the formation of fine bubbles and also to ensure that a stable froth is formed as the bubbles rise and disengage from the liquid.
In the flotation cell, the values adhere to the bubbles, which carry them to the surface and into the stable froth layer. The froth discharges over the lip of the cell, carrying the values. The waste gangue remains in the liquid in the cell and is discharged with the liquid to a tailings disposal facility. The primary purpose of the flotation process is to separate or remove selected particles, that are either naturally hydrophobic or can be caused to be hydrophobic by appropriate addition of reagents (conditioning), from a mixture of hydrophobic and non-hydrophobic particles (mixed particles), in a suspension in water.
The formation of a froth layer is an important characteristic of the froth flotation process. In a stable froth layer, froth is discharged over the lip of the flotation cell, being continuously replaced by bubbles with attached particles, and entrained particles, from the pulp or slurry in the cell beneath. While moving towards the overflow lip, the froth drains and entrained particles are able to flow back into the pulp, enhancing the purity or grade of the flotation product.
It is recognised that there is a limit to the size of particles that respond well to flotation. Above a certain size, which is of the order of 100 microns for particles of base metal sulfides, or 350 microns for coal particles, the recovery of particles in a flotation cell decreases, as the particle size increases. We refer to such particles as “coarse” particles.
It is well established that coarse particles are difficult to float because of the effect of turbulence in the flotation machines in current use. In mechanical cells, the particles are kept in suspension by the action of a rotating impeller in the base of the cell. The impeller is also used to disperse an air flow into bubbles which are essential for the flotation process. By its very nature, the impeller causes the motion of the fluid in the cell to be highly turbulent in nature, characterised by the existence of vortices or eddies with a wide range of diameters and rotational speeds. In flotation columns, turbulent motions arise from convection currents established by bubbles rising through the liquid in the column. In both these examples, when a bubble is trapped in the centre of an eddy, it will rotate at the rotational frequency of the eddy, and if a large particle above a certain critical size is attached to the bubble, it will be flung away by centrifugal force that ruptures the bubble-particle aggregate. A theory exists for calculating the maximum floatable diameter of a particle with known physical properties (Schulze, H J (1977). New theoretical and experimental investigations on stability of bubble/particle aggregates in flotation: a theory on the upper particle size of floatability. Int. J. Miner. Process., 4, 241-259. See also Schulze H J (1982). Dimensionless number and approximate calculation of the upper particle size of floatability in flotation machines. Int. J. Miner. Process., 9, 321-328.)
It is clear that existing technologies have a severe limitation in regard to their ability to recover coarse particles. There is a need for a way of conducting flotation that substantially eliminates turbulence from the environment in which the capture of particles by bubbles is performed. It is an object of the present invention to reduce turbulence in a flotation cell.
A number of terms relating to the phenomenon of fluidization are now defined, with reference to a vertical cylindrical column, containing solid particles and a liquid such as water. A stream of liquid containing particles in suspension flows upwards in the column, being distributed uniformly across the entry plane at the base. The feed flowrate is kept constant, while the diameter or cross-sectional area of the column is allowed to change. The concentration of particles in the feed stream is such that the particles are free to move relative to each other, and the volume fraction of particles in the feed is lower than the volume fraction of solids in a packed bed, which is typically of the order of 0.4. (A packed bed forms when solids are allowed to settle in a stationary liquid layer in the column, i.e. where there is no entry of fresh liquid.) When the area of the column is large, the upward velocity of the liquid is very low, and the particles settle against the rising liquid. (The velocity here is the superficial velocity, which is the volumetric flowrate of liquid (or water or solid particles as appropriate) divided by the horizontal cross-sectional area of the column.) A bed of particles, in which each particle is supported by the adjacent particles with which it is in contact, moves slowly up the column. This is referred to as a moving bed. If the column area is further reduced, the particles in the bed still tend to settle against the upward flow of liquid in the feed stream. Across the bed in the vertical direction, a frictional pressure drop is created due to the relative velocity between the particles and the liquid. At a certain liquid velocity, the pressure drop becomes sufficient to support the effective mass of all the particles, so that each particle is supported by the upward motion of the liquid, rather than by the adjacent particles. The superficial liquid velocity at which this occurs is referred to as the minimum fluidization velocity. With further reduction in column area, the particles move further apart. The volume fraction of solids is less than that in a packed bed, and an expanded fluidized bed or expanded bed is created. As the column area is reduced still further, the solids volume fraction decreases further, until it equals the volume fraction in the feed flow. In a related phenomenon in a fluidized bed where there is no net inflow of particles, when the liquid velocity is less than the terminal velocity of the particles, they will stay in the enclosing vessel and a static bed is formed, which may or may not be in an expanded state. When the upward liquid velocity exceeds the terminal velocity of the particles, they are entrained into the flow, the basis of the process known as elutriation.
An important concept in fluidization studies is that of slip, by which is meant the difference in the superficial velocities of the suspending fluid and the solid particles. Consider the system above in which there is a continuous feed of solids and water to the column. The feed is relatively dilute, so the volume fraction of solids is much less than the volume fraction that would exist in a packed bed of the same solids. If there is a large superficial velocity difference between the solids and the liquid, giving a high slip velocity, the particles will accumulate in the bed, and the solids volume fraction will increase, with corresponding drop in liquid fraction. The liquid fraction represents the fraction of the cross-section of the bed that is available for the through-flow of the liquid. Thus an increase in solids fraction leads to a reduction in the flow area available to the liquid, and hence to an increase in the drag force exerted on the particles which leads ultimately to the formation of a fluidized bed. In a steady state operation, the solids fraction in the bed when it is fluidized will be higher than the solids fraction in the feed flow. When the particles are very small so that their terminal settling velocity is much lower than the liquid velocity in the bed, there will be very little slip between the liquid and the particles, so the solids fraction in the column will be essentially the same as the solids fraction in the feed. Such a flow in the column is referred to as a co-current flow. In a co-current flow, all the particles in the suspension flow upwards with the liquid.
A spouted bed is a bed of particles through which a vertical rising jet of fluid is injected centrally through the base of the bed. To form a spout, the entering fluid must exceed a minimum spouting velocity. In steady-state operation, a circulation pattern is established in the bed in which the solids entrained by the fast-moving entrance jet rise upwards. If the bed is relatively shallow, the jet actually penetrates the upper surface of the bed, and particles rise above this surface and fall back on the annular area surrounding the jet. If the bed of particles is deep, a recirculating spouted bed may form in the base of the bed, and rise to a certain height (the maximum spout height) before its energy is spent, and a normal fluidized bed forms above the spouted zone. Spouted beds may form in a simple right cylinder with a flat base, in a right cylinder with a conical base, or in a cone.
For purposes of this specification, liquid generally has the meaning of a liquid alone, such as water, or it may on occasion refer to a dilute suspension of solids in water. A concentrated suspension of particles in a supporting liquid such as water is referred to as a slurry or pulp. If a pulp is flowing in a pipe at a certain flowrate, it is clear that there will be corresponding flowrates of the constituent components, the liquid and the solids. Where it is necessary to distinguish between the liquid and the solids in a feed or a fluidized bed, the liquid component of the slurry will be described as water. Fluid has the meaning of anything that flows, including a gas such as air, a liquid such as water, and a suspension of particles in a liquid, such as the feed suspension of particles that is fed to a flotation cell. Because of the slip that exists in a fluidized bed, the superficial velocity of the particles in the bed relative to space is generally different to that of the supporting liquid, which is generally water.
There are a number of prior inventions that have attempted to improve the recovery of coarse particles in flotation. McNeill (U.S. Pat. No. 4,960,509) modified a mechanical flotation cell by the incorporation of a vertical baffle that divided the cell into two compartments, a feed zone and a flotation zone. A pulp of crushed ore suspended in water passes from the feed zone through an impeller where it is brought into contact with air bubbles. The aerated pulp then rises through a perforated plate towards the top of the cell, where the bubbles disengage from the liquid and pass into the froth layer, carrying any attached particles with them. The impeller in the cell has the dual function of breaking up the air stream into small bubbles, and also of keeping the particles in the feed in suspension, so that they do not sediment in the bottom of the cell. This device suffers from an important deficiency in relation to the flotation of coarse particles, since it depends on the suspending action of the impeller, which will inevitably introduce high energy-dissipation rates throughout the flotation cell, and create high levels of turbulence that will cause coarse particles to detach from the bubbles. To maximise coarse particle recovery it is preferable to do away with rotating impellers or any device that will create high levels of turbulence in locations where such particles can be detached from bubbles. It is an object of the present invention to create an environment that is conducive to capture and retention of coarse particles and which does not require mechanical agitation.
U.S. Pat. No. 6,425,485 (Mankosa et al) describes a hydraulic separator in which the density of one type of particle is decreased by the adherence of air bubbles, thereby facilitating the separation of such particles from others of higher density, in a fluidized bed separator. The invention is in effect an extension of a device in common use for gravity separation, known as the teeter bed separator. A feed containing particles in suspension is introduced near the top of a rectangular cell. Provision is made to withdraw solids and liquid from a dewatering cone at the base of the cell, and also from a collection launder at the top of the cell. A fluidized bed known as a teeter bed forms in the cell, so that particles whose density is less than the average density of particles in the bed float to the top. The teeter bed is fluidized with fresh water, into which air bubbles are injected. The bubbles attach to any particles in the bed that are hydrophobic, and carry them to the surface of the vessel and into the collection launder, along with any materials of low density that may exist in the feed. The device is described in terms of its ability to separate particles on the basis of their density. However, this invention has severe limitations if used for flotation. As noted, there are two slurry discharge streams, one out of the bottom of the cell and the other out of the top. Whether or not there are hydrophobic particles in the feed to the cell, the lighter particles will be removed at the top of the vessel. If the feed contains hydrophobic particles that will attach to bubbles, they too will flow out of the top of the vessel, mixed with low-density hydrophilic particles. In flotation, it is desired to separate the hydrophobic particles from the hydrophilic particles, and the Mankosa device cannot do this. The inability to distinguish between particles that arrive in the collection launder because they are of lower density than those in the underflow discharge, and those that are present because they are hydrophobic and have become attached to air bubbles, is a very severe limitation from the point of view of the flotation process. Another weakness of this invention is the necessity to use clean water as the fluidizing fluid. In many mining locations, water is scarce and costly and it is desirable to minimize the clean water requirements of any mineral processing operation.